Soft-nitriding procedure for steel and cast iron

ABSTRACT

A METHOD OF INCREASING THE NITROGEN CONTENT OF STEEL AND CAST IRON WHEREIN THE BODY IS MAINTAINED IN AN ATMOSPHERE OF AMMONIA, METHANE AND OXYGEN IN A MOLAR RATIO OF CH4:NH3=1 AND (CH4+NH3):O2=25-30 WHILE MAINTAINING THE DISSOCIATION RATE OF THE AMMONIA BETWEEN 38 AND 40%. THE TREATMENT TIME IS 1 TO 10 HOURS AT A TEMPERATURE OF 570* C.+ 10* C.

United States Patent f 3,705,058 SOFT-NITRIDING PRQCEDURE FER STEEL AND CAST IRON Zoltan Kolozsvari and Endre Gal, Targu-Mures, Rumania, assignors to Fabrica de Utilaie si Piese de Schirnb Metalotehnica, Targu-Mures, Rurnania No Drawing. Filed July 7, 1971, Ser. No. 160,523 Claims priority, application Rumania, July 9, 1970,

6 Int. Cl. c236 11/16, 11/18 U.S. Cl. 148-16.6 1 Claim ABSTRACT OF THE DISCLOSURE A method of increasing the nitrogen content of steel and cast iron wherein the body is maintained in an atmosphere of ammonia, methane and oxygen in a molar ratio of CH :NH =1 and (CH,+NH ):O =25-30 while maintaining the dissociation rate of the ammonia between 38 and 40%. The treatment time is 1 to hours at a temperature of 570 C. :10 C.

SUMMARY As a result of the treatment a compound layer of a thickness of 10-15 m. is obtained, supported by a diffusion layer with a case depth depending on treating time and material composition.

The invention refers to a soft nitriding (low temperature carbonitriding) procedure of steel and cast iron in a controlled atmosphere.

Soft-nitriding of steel and cast iron machine parts is generally performed in a salt bath containing alcaline cyanides and cyanates. The active carbon and nitrogen appears as a result of decomposition of the above salts. The working temperature of the salt baths is of 570:10" C. (below of critical temperatures in Fe-N system). As result of the treatment a surface layer is to be obtained, composed of a compound (white) layer and a diffusion layer. The compound layer is responsible for the high resistance to wear and scufiing, the diffusion layer forms a support.

The above procedure presents disadvantages, because applying a salt bath with heavy toxic effect and by the other hand no continuous inspection and control of the bath characteristics is possible, the only inspection is by performing chemical analyses.

A soft-nitriding procedure in a gaseous media is also known, using Endo-gas and ammonia. Backdraw of this procedure is the use of gas generator and a low nitriding potential.

The procedure in conformity with the invention eliminates the above disadvantages, performing the nitriding in order to realize a controlled and uniform treatment, at 570 i 10 C., in an atmopshere composed of ammonia, methane and oxygen. The ratio of gas components is as Patented Dec. 5, 1972 follows: CH /NH =1 and (CH +NH )/O =25-30, with an ammonia dissociation rate of 384-0%, and a treating time of 1-10 hours.

An example of realizing the invention is given below, in order to soft-nitride pieces of OLC 35 (plain carbon steel with 0.35% C).

The treatment has been performed in a gaseous media of the following composition: NH /CH1,=1, adding oxygen in a ratio of CH NH /O =25-30, at a treating temperature of 570 :10 C. The ammonia flow-rate has to be adjusted for a dissociation degree of 35-40%.

A three hours treatment assures the following results:

Hardness Sample HB HVI HV3 HVfi HV1 HVt! Untreated 200-207 Nltrlded in salt bath 510 440 407 370 320 Gas-nitrided 490 430 400 380 340 Metallographic examination has shown the following data:

case depth (of diffusion layer): 0.350.40 mm. depth of compound layer: 10-15mm. microhardness of compound layer: 870-920 MHV.

On the basis of experiments the parameters of the above treatment, in conformity with the invention, have been established, as follows:

(a) temperature: 570 :10 C. (b) ratio of the components: NH /CH =1 and (CH-I-NHS (c) ammonia dissociation rate: 35-40% (d) treating time can vary between 60 min. and 10 hours depending on parts to be treated and material.

Advantages of the procedure in conformity with the invention are:

a clean metal surface can be obtained all the parameters of the procedure can be precisely checked and controlled.

What is claimed is:

1. A soft-nitriding procedure for increasing the nitrogen content of steel and cast iron articles, comprising heat treating said articles for 1 to 10 hours at a temperature of 570 C. :10 C. in an atmosphere of ammonia, methane and oxygen while maintaining the dissociation rate of the ammonia between 38 and 40%, said atmosphere being characterized by having molar ratios of CH; to NH;, of 1 and CH,+NH to 0 of 25 to 30.

References Cited UNITED STATES PATENTS 3,228,807 1/1966 Mitchell et al. 148-165 X 3,519,257 3/1968 Winter et al. 148-16'.5 X 3,620,518 11/1971 Winter et al. 148--16 3,399,085 8/1968 Knechtel et al. 148-16.6

FOREIGN PATENTS 125,006 Czechoslovakia.

48,847 Germany.

CHARLES N. LOVELL, Primary Examiner U.S. Cl. X.R 148-165 

